Particle accelerators for accelerating ions and protons are being widely utilized for physical experiment and cancer therapy apparatus. The related background art for the present invention will be described below in terms instances where a particle accelerator is employed for the cancer therapy apparatus. Proton beams and heavy particle beams are being employed for cancer therapy. More specifically, the related background art will be described below in terms of heavy particle beams. Heavy particle beams are mainly formed by carbon ions. Carbon ions are generated from an ion source and accelerated by a plurality of accelerators before they are irradiated onto the diseased part of a patient (see, for example, Japanese Patent Application Laid-Open Publication No. 2009-217938, Japanese Patent Publication No. 2,596,292 and Japanese Patent Publication No. 3,246,364 the entire contents of which are incorporated herein by reference).
An ion accelerator comprises as main components thereof an ion source, a linear accelerator (radio frequency quadrupole type linear accelerator, to be referred to as RFQ hereinafter), a drift tube type linear accelerator (drift tube LINAC, to be referred to as DTL hereinafter), a beam transport system and a synchrotron.
In conventional ion accelerators for cancer therapy, quadrivalent carbon ions (C4+) are generated in an electron cyclotron resonance (to be referred to as ECR hereinafter) ion source. The generated quadrivalent carbon ions are accelerated to get to an energy level of several MeV/u by a linear accelerator and subjected to charge conversion in a charge converter to become hexavalent carbon ions (C6+). The hexavalent carbon ions (C6+) are then injected into a synchrotron so as to be accelerated by the synchrotron.
Essentially, linear accelerators show a high acceleration efficiency when they use hexavalent carbon ions (C6+). However, conventional ECR ion sources cannot reliably provide the amperage of hexavalent carbon ions (C6+) that is required for cancer therapy. Therefore, conventional ECR ion sources use quadrivalent carbon ions (C4+).
The ion beam coming out from the ECR ion source is a DC beam and there is an upper limit amperage for the ion beam that can be extracted from an ion source (currently several hundred μA). For this reason, a so-called multi-turn injection technique is employed to inject an ion beam into a synchrotron to secure the number of ions necessary for cancer therapy.
Meanwhile, single turn injection and multi-turn injection are known to date as techniques for injecting an ion beam into a synchrotron. With the single turn injection technique, the number of ions necessary for cancer therapy is injected with a one-time injection. On the other hand, the multi-turn injection technique is a technique of securing the number of ions necessary for cancer therapy by causing injected ions to travel along a circulating path and injecting additional ions for a plurality of times.
However, when injecting a beam after the circulation time (about 2 μsec) is over, there arises a problem that the newly injected ions cannot follow the path of the ion beam that has made a full turn once. To avoid this problem, the circulating path of the ion beam in the synchrotron is shifted by using a bump magnet to make the path vary with time and allow additional ion beam injections for a plurality of times.
Besides, for accelerator systems that are currently popularly being operated in Japan, the optimum energy for converting carbon quadrivalent ions (C4+) into carbon hexavalent ions (C6+) is 4 MeV/u so that the level of acceleration energy in linear accelerators is determined.
Meanwhile, a large electric current can be extracted from a laser ion source although only with short pulses (up to several μsec). A laser ion source is an apparatus designed to condense a laser beam, irradiating it onto a target and, evaporating and ionizing the target element by means of the energy of the laser beam to generate plasma. The ions contained in the plasma are transported as plasma ions and accelerated when they are extracted to produce an ion beam (see, for example, Japanese Patent Application Laid-Open Publication No. 2012-99273 the entire content of which is incorporated herein by reference).
A laser ion source can produce an ion beam by irradiating a laser beam onto a target and generate ions. It is advantageous for generating a high current multivalent ion beam. Reports say that hexavalent carbon ions (C6+) generated by laser ion sources can produce peak electric currents in the order of several mA with a pulse width up to 2 μsec in synchronism with the timing of laser irradiation. The result corresponds to the number of ions with which the synchrotron of a cancer therapy can provide the required amperage in a single pulse.
Now, a known ion accelerator will be described below by referring to FIG. 7.
As shown in FIG. 7, the ions generated by an ion source 1 are transported by means of a low energy beam transport system (to be referred to as LEBT system hereinafter) 2 to an RFQ 3 and a DTL 4, which are linear accelerators arranged downstream, while the beam characteristics thereof are regulated by the LEBT system 2. A known technique of causing electric discharge to occur in gas to obtain ions is employed for the ion source 1. A microwave or an electron beam is utilized to cause electric discharge to occur.
Generally, ECR ion sources are employed in accelerators for cancer therapy. ECR ion sources are designed to ionize gas in order to generate plasma and extract ions by means of an electric field. The extraction current is a direct current. While ECR ion sources can generate multivalent ions, high valent ions show only a small amperage value. Therefore, the ECR ion source generates quadrivalent carbon ions (C4+) in order to secure the ion amperage required for cancer therapy and then accelerate the carbon ions by means of the RFQ 3 and the DTL 4.
The ions emitted from the DTL 4 are converted from quadrivalent carbon ions (C4+) to hexavalent carbon ions (C6+) by a charge converter 5 and transported into a synchrotron 7 by way of middle energy beam transport system (to be referred to MEBT system hereinafter) 6.
The synchrotron 7 comprises deflector electromagnets 8, quadrupole electromagnets 9, hexapole electromagnets 10 and a radio frequency accelerating cavity 11. Although not shown, the synchrotron 7 additionally comprises a correcting magnet and monitors for monitoring ion beams. After the ion beam is accelerated to get to a satisfactory energy level, it is transported to an irradiation room (not shown) by way of an injection path 13 that passes a bump electromagnet 12 for ion beam injection and a septum electromagnet (not shown) and irradiated to the diseased part of a patient for cancer therapy.
Generally, the ion beam that is injected into the synchrotron 7 is provided with an injection path prepared by using the bump electromagnet 12 for ion beam injection that is arranged in the synchrotron 7 for the purpose of multi-turn injection.
Conventional multi-turn injection method will be described below in terms of proton beam multi-turn injection that disclosed in OHO '87 High-energy Accelerator Seminars shown in FIG. 8. As shown in FIG. 8, the bump electromagnet 12 for ion beam injection shifts the path each time the ion beam makes a full turn. The ion beam is forced to make full turns by way of the deflector electromagnets 8, the quadrupole electromagnets 9, the hexapole electromagnets 10 and the septum magnet 18 that are the magnets of the synchrotron so as to be accelerated to a predetermined energy level.
The magnetic excitation waveform of the bump electromagnet 12 is shown in FIG. 9. Ion beam injections are conducted at the side where the extent of magnetic excitation falls. While the magnetic excitation width depends on the ring circulation time in the synchrotron 7, it is in the order up to several hundred μsec. Therefore, the ion current that is generally employed for injection is subjected to chopping (of removing the beam that becomes unnecessary for a certain period) before being injected into the RFQ 3 in synchronism with bump excitation time.
As described above, since the highest amperage value of the electric current of the particle source for a particle accelerator in a synchrotron is relatively small, the number of particles necessary for cancer therapy or a physical experiment is obtained by means of multi-turn injection.
For this reason, conventional particle accelerators face a problem that it is difficult for them to raise the amperage value of the circulating current. It is difficult in the current status of technology to remarkably raise the amperage value of the circulating current although it is desired for scanning irradiations and other applications.
A remarkable improvement can be expected for the amperage value of the circulating current when the technique of multi-turn injection using a short pulse particle source showing a large highest peak current value such as a laser ion source is employed. However, since the pulse width of the laser ion source is equal to or smaller than the synchrotron circulation time, it has not been possible to realize multi-turn injection with the conventional method.
Therefore, the object of the present embodiment is to provide a particle accelerator for which the amperage value of the circulating current can be raised and the particle beam utilization efficiency can be improved and also provide medical equipment using such a particle accelerator in order to solve the above-identified problem.